The present invention relates to an apparatus for computerized tomography (called an apparatus of CT hereafter), especially to an apparatus of CT for an object having large density difference, which is preferable for reducing radial false image (called artifact hereafter) appearing in a reconstructed image, and a method of same.
An apparatus of CT is an apparatus for irradiating an object with waves such as electromagnetic wave, ultrasonic wave, and light wave from a radiation source, detecting penetration data to show strength of penetration of the wave with a detector, obtaining the penetration data from every direction, processing a plurality of the penetration data, and reconstructing sliced image of the object. For obtaining the penetration data from every direction by the apparatus of CT, the detector is installed facing to the radiation source with a separation at midst of which the object is inserted, and at least one of the detectors and the radiation source is rotated and scanned at regular intervals and obtain a series of penetration data in a range at every rotation. A series of the penetration data is called translational data hereafter.
With the apparatus of CT described above, if the object contains a foreign body having high spatial frequency component (for example, existing of a body shaped to have sides like as quadrilateral, triangle, and needle-shape, etc.) and large density difference from another region, a radial artifact is generated from a corner part of the side. The artifact is not existing actually in the object, and affects not only the generating point but also other part of the object. The generation of artifact is regarded as having different density from surroundings, and density resolving power at the point where the artifact generated becomes worse. Especially reconstructed image is blurred at the region of sides of the foreign body, in other words, spatial resolving power becomes worse.
To solve more or less the problem described above, there is a method as a prior art to affect a high frequency shielding type spatial filter when reconstruct image from a plurality of penetration data and to flatten the reconstructed image itself.
By the prior art described above, there was a problem that other part was also flattened concurrently and spatial resolving power of the other part became inferior although the aimed artifact is reduced.
Also, considering that an essential cause to yield artifacts is obtaining penetration data dispersedly, a method to increase number of the penetration data in order to close the penetration data as continuous as possible is thought. But the method increases scanning time of a detector to obtain a reconstructed image and is not adequate for an apparatus of CT which has an essential requirement to shorten scanning time. Further, if high energy radiation is used, large pitch of detectors are necessary by an effect of dispersed radiation at the detectors, and it is impossible to increase number of the penetration data.